In recent years there has been a general trend to reduce the size of electronic devices such as mobile communications devices. The functional circuitry within these devices is generally mounted on a printed wiring board (PWB). The size of the PWB is limited by the size of the device.
In electronic devices which comprise antennas the PWB is often used as the ground plane for the antenna arrangement. The ground plane and the antenna both act as resonators so that the input impedance and the radiation pattern of an antenna arrangement are defined by the distribution of current within the ground plane and the antenna element. Therefore the ground plane must be of a particular length so that it can resonate at the correct frequencies. If the ground plane becomes too short then this has adverse effects on the operation of the antenna arrangement.
For example at 900 MHz (EGSM operation) the optimal ground plane length would be between 80-120 mm. However many mobile telephones are much smaller than this and the ground plane may be as short as 40 mm which is too short to resonate at 900 MHz.
It would be desirable to provide an antenna arrangement which is operable at the lower frequencies even if the PWB is too short.